Such a transmission system is disclosed by the paper entitled "Polarization-Switching Techniques for Coherent Optical Communications" which appeared in Journal of Lightwave Technology, Vol.6, No.10, October 1988, pages 1537 to 1548.
It is generally known that, at the end of an optical waveguide or optical fiber, the polarization state of the light transmitted over the optical waveguide fluctuates. Various causes for this phenomenon can be identified, such as a mechanically twisted optical fiber, temperature variation, bends in the optical fibers and the like. The varying mismatching between the polarization of the received signal and the polarization of the signal from the local oscillator adversely affects the sensitivity of the receiver. In the paper mentioned above, a number of solutions are mentioned for obtaining a transmission over an optical fiber which is as insensitive as possible to the varying polarization. One of the solutions is the polarization-switching method, in which a high-order retarder making use of a birefringent medium is used.
A medium having a large birefringence effect can be simulated in a laboratory by means of optical devices. In these, light is applied to a polarization-splitting device, as a result of which the light is split into components having horizontal and vertical polarization, respectively. Said components traverse different distances and are then combined in a coupling device. One problem in this connection is that two polarization-splitting devices are necessary which, in a bulk or optical fiber construction lead to a high price, while, in an integrated optical construction, the total length of the high-order retarder can be a problem. An alternative method of construction is to use a birefringent fiber, as described in the abovementioned article. Such a fiber having birefringence has, however, the disadvantage that it is expensive while, in addition, a large length is necessary.
The paper by M. V. Andres et al. entitled "Optical-fiber resonant rings based on polarization-dependent couplers", published in Journal of Lightwave Technology, Vol. 8, No. 8, August 1990, pages 1212-1220 discloses sensor configurations based on an optical fiber resonant circuit in which, in a particular case, a polarization-splitting coupling is used in combination with a loop of polarization-maintaining fiber.